(

)

2

1

12

2

OUT

IN(MAX)

OUT

L(RMS)

OUT(MAX)

IN(MAX)

OUT

SW

V

V

V

I

I

V

L

F

0.8

´

-

æ

ö

=

+

´ ç

÷

´

´

´

è

ø

OUT

IN(MAX)

OUT

L(PK)

OUT(MAX)

OUT

IN(MAX)

SW

V

(V

-V

)

I

I

1.6

V

L

F

´

=

+

´

´

´

OUT MIN

OUT

CO

K

C

L

2

(

)

æ

ö

1

=

´ ç

÷

2p¦

è

ø

)

OUT

IN(MAX)

OUT

COUT(RMS)

IN(MAX)

OUT

SW

C

1

V

(V

V

I

V

L

F

N

12

´

-

é

ù

=

´ ê

ú

´

´

´

ë

û

(

)

æ

ö

´

´

´

´

´ D

ç

÷

ç

÷

´

è

ø

IN(MAX)

OUT

SW

MAX

C

p-p(MAX)

OUT

IN(MAX)

OUT

V

L

F

0.8

ESR

=N

V

V

V

-V

TPS54110

SLVS500C

– DECEMBER 2003 – REVISED FEBRUARY 2011

www.ti.com

(5)

The peak inductor current is determined from Equation 6:

(6)

For this design, the RMS inductor current is 1.503 A and the peak inductor current is 1.673 A. The inductor

chosen is a Coilcraft DS3316P-682 6.8

µH. It has a saturationcurrent rating of 2.8 A and an RMS current rating

of 2.2 A, easily meeting these requirements.

Capacitor Selection

The important design parameters for the output capacitor are dc voltage, ripple current, and equivalent series

resistance (ESR). The dc-voltage and ripple-current ratings must not be exceeded. The ESR rating is important

because along with the inductor current it determines the output ripple voltage level. The actual value of the

output capacitor is not critical, but some practical limits do exist. Consider the relationship between the desired

closed-loop crossover frequency of the design and LC corner frequency of the output filter. In general, it is

desirable to keep the closed-loop crossover frequency at less than 1/5 of the switching frequency. With high

switching frequencies such as the 700 kHz frequency of this design, internal circuit limitations of the TPS54110

limit the practical maximum crossover frequency to about 100 kHz. To allow adequate phase gain in the

compensation network, set the LC corner frequency to approximately one decade below the closed-loop

crossover frequency. This limits the minimum capacitor value for the output filter to:

(7)

10 for one decade of difference.

For a desired crossover of 60 kHz, K=10 and a 6.8

μH inductor, the minimum value for the output capacitor is

100

μF. The selected output capacitor must be rated for a voltage greater than the desired output voltage plus

one half the ripple voltage. Any derating factors must also be included. The maximum RMS ripple current in the

output capacitor is given by Equation 8:

(8)

where NC is the number of output capacitors in parallel.

The maximum ESR of the output capacitor is determined by the allowable output ripple specified in the initial

design parameters. The output ripple voltage is the inductor ripple current times the ESR of the output filter so

the maximum specified ESR as listed in the capacitor data sheet is given by Equation 9:

(9)

For this design example, a single 100

µF output capacitor is chosen for C2. The calculated RMS ripple current is

80 mA and the maximum ESR required is 87 m

Ω. An example of a suitable capacitor is the Sanyo Poscap

6TPC100M, rated at 6.3 V with a maximum ESR of 45 milliohms and a ripple-current rating of 1.7 A.

Other capacitor types work well with the TPS54110, depending on the needs of the application.

Compensation Components

The external compensation used with the TPS54110 allows for a wide range of output-filter configurations. A

large range of capacitor values and dielectric types are supported. The design example uses type 3

compensation consisting of R1, R3, R5, C6, C7 and C8. Additionally, R2 and R1 form a voltage-divider network

that sets the output voltage. These component reference designators are the same as those used in the SWIFT

Designer Software.

10

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